The present invention relates to a chemical filter unit and a gas cleaning system.
The present invention relates to a chemical filter unit and a gas cleaning system which can decrease the chemical pollutants in a gas, particularly in the air supplied to a clean room, clean booth or clean bench, etc. (hereinafter called a closed clean space) to very low levels by using ion exchange or electric charge adsorbing action, are low in the pressure loss of the gas to be treated and can be used for a long period.
In recent years, in the electric and electronic industry, biochemical industry, etc., the demand for closed clean spaces has been sharply increasing. With the improvement of production techniques in the respective industries, the demand for cleanness becomes very severe. In this connection, techniques for analyzing pollutants are also improving dramatically in these years. As a result, the necessity of removing conventionally overlooked substances has been noticed.
The conventional cleaning of air in closed clean spaces is to mainly remove solid particulates by using particulate removing filters such as HEPA filters, and the removal of solid particulates is now controlled to the greatest possible extent.
However, it has been found that when the air in a closed clean space is used cyclically through filters, some of the misty or gaseous chemical substances contained in the air are rather concentrated to levels higher than those in open air. The reason is that in the conventional cleaning of air, the necessity of removing misty or gaseous chemical substances contained in the air has not been noticed, and therefore that the conventional air cleaners did not have such a function.
In the electronic industry where products like very large scale integrated circuits are produced, the necessity of removing the chemical substances began to be noticed. Especially in the photolitho process, the phenomena that the yields of products are lowered by the presence of these chemical substances have been found, and the necessity of developing chemical substance removing techniques of a concept quite different from that adopted in the conventionally used HEPA filters has been noticed.
Especially it is demanded to develop techniques for removing ammonia and amines such as organic amines. Ammonia causes haze and dielectric breakdown in the product, and its presence greatly lowers the yield of the product.
In the chemically amplified resist production process of the photolitho process, protons are generated in the regions irradiated with light as a chemical amplifying agent, to promote the dissolution into the developer. According to this mechanism, the device is formed. If ammonia exists in the atmosphere, protons are neutralized, and dissolution into the developer in this portion is inhibited. This causes a very serious problem called T-TOP failure.
This phenomenon occurs when the ammonia concentration in the air is as small as several parts per billion. The ammonia concentration of the air to be treated is usually tens to hundreds of parts per billion, though different from place to place. To solve the problem, it is necessary that the ammonia concentration in the cleaned air is 5 ppb or less.
LSI manufacturers are required to solve this problem, and desire the development of any filter unit which can keep the ammonia concentration in the air of a closed clean production space as very low as possible.
Japanese Patent Laid-Open No. 1-317512 discloses a method for removing sea salts (including ammonia) in air by letting the air pass through a filter medium containing ion exchange fibers. However, this technique does not state any possibility of removing ammonia by adsorption down to a very low concentration (order of ppb), or any method for lessening the pressure loss of the filter using the filter medium, or any method for increasing the substance adsorption capacity per unit volume of the filter medium which indicates the filter service life.
Japanese Patent Laid-Open No. 61-138543 discloses a filter medium obtained by corrugating a laminate formed by overlaying pulp sheets on both sides of an ion exchange fiber-containing sheet. However, the filter medium is used in the crossflow method in which a gas is fed from the front sheet face of the filter medium to the back sheet face. The technique does not state any method for lessening the pressure loss of the filter using the filter medium or any method for increasing the substance adsorption capacity per unit volume of the filter medium which indicates the filter service life.
Japanese Patent Laid-Open No. 60-183022 discloses a filter composed of cation exchange fibers for arresting mutagenic substances in air. However, the filter medium is used in the crossflow method. The technique does not state any method for lessening the pressure loss of the filter using the filter medium or any method for increasing the substance adsorption capacity per unit volume of the filter medium which indicates the filter service life.
Japanese Laid-Open No. 8-24564 discloses a filter unit containing ion exchange fiber sheets. However, the technique does not state any method for lessening the pressure loss of the filter in the filter unit or any method for increasing the substance adsorption capacity per unit volume of the filter medium which indicates the filter service life.
Therefore, the above prior art cannot meet the demand in the above technique field.
The object of the present invention is to provide a filter unit and a gas cleaning system capable of responding to the above demand.
The present invention provides a filter unit with a very large adsorption capacity of the filter medium compared to the conventional filter media and small in the pressure loss of the filter medium compared to the conventional filter media.
To solve the above problems, it has been considered to increase the quantity of the filter medium per volume, but this method does not solve the latter problem. To solve the latter problem, it has been considered to decrease the quantity of the filter medium per volume, but this method does not solve the former problem. The present invention solve these contradictory problems all at once.
A chemical filter unit, which comprises a filter medium formed by laminating a plurality of fiber sheets and a housing for containing the filter medium, and has a gas inlet open on one face of the housing and a gas outlet open on the other face substantially opposite to the gas inlet, characterized in that gas passages to allow the flow of the gas along the surfaces of the fiber sheets are formed between the respectively adjacent fiber sheets of the filter medium from the gas inlet to the gas outlet, and that the adsorption capacity of the filter medium is 300 eq/m3 or more.
The gas passages allow the raw gas to flow mainly along the surfaces of the fiber sheets constituting the filter medium. This means that the flow of the raw gas through the filter medium is parallel flow. Since this structure is formed substantially by fiber sheets only, the adsorption capacity of the filter medium can be kept at 300 eq/m3 or more and the pressure loss of the filter medium can be kept small.
The adsorption capacity refers to the amount of plus charged particulates, mist and gas adsorbed per unit volume (1 m3) of the is filter medium under chemical balance. For example in the case of active carbon fibers carrying phosphoric acid, the adsorption capacity is calculated from the total weight of the carried phosphoric acid, based on the maximum balancing capacity achieved when all the reactive groups have reacted to form salts.
In the case of ion exchange fibers, the ion exchange capacity is the adsorption capacity. The method for measuring the ion exchange capacity is not specified. In general, a filter medium with a certain capacity is cut off and caused to react in an acid or alkali with a known normality, and the residue is neutralized for titration.
A chemical filter unit, according to the first version, wherein when the average flow velocity at the gas inlet, of the gas flowing from the gas inlet to the gas outlet is 0.5 m/sec, the pressure loss of the flowing gas by the filter medium with a depth of 70 mm in the gas passage direction is 3 mm Aq or less.
A chemical filter unit, according to the first or second version, wherein the fibers constituting the fiber sheets are chemically modified active carbon fibers (for example, active carbon fibers carrying phosphoric acid) or ion exchange fibers, preferably ion exchange fibers.
The ion exchange fibers used here will be described later in detail.
A chemical filter unit, according to any one of first to third versions, wherein the filter medium is immobilized in the housing substantially only by the resiliency of the plurality of fiber sheets.
That the filter medium is immobilized substantially only by the resiliency of the plurality of the fiber sheets means that when the filter unit is used, the means for laminating the fiber sheets and immobilizing the filter medium in the housing do not evolve any gas to decrease the effect of gas cleaning. For example, if the fiber sheets are laminated using an adhesive or supported in their form by any other organic material or if the filter medium is immobilized in the housing using an adhesive, then any gas to decrease the effect of air cleaning may be evolved.
Other versions of the present invention will be described below in more practical explanation with respect to the present invention.
It is preferable that the fiber sheets are formed by scooping short fibers of 0.1 to 10 mm in length, such as filter paper using a wire cloth. It is preferable that the a real unit weight of the fibers of the fiber sheets is in a range of 30 to 1000 g/m2, and a more preferable range is 50 to 500 g/m2. If the fiber sheets are post-processed by folding or thermal bonding, etc., an especially preferable range is 100 to 300 g/m2.
The fiber sheets may also have any other material than the chemically modified active carbon fibers and ion exchange fibers mixed. For improving paper formability, long fibers of cellulose or pulp, etc. may be mixed to some extent. Inorganic fibers can also be mixed. When other materials are mixed, it is preferable that the amount of the chemically modified active carbon fibers or ion exchange fibers is 50 wt % or more.
It is important that the filter medium is a three-dimensional structure formed by a laminate comprising a plurality of fiber sheets, and that gas passages from one face of the filter medium to the corresponding other face are formed between the adjacent fiber sheets.
The gas passages can be formed by laminating undulating fiber sheets. For example, corrugated structure, honeycomb structure, simply wavy structure and their combinations, etc. can be used. Corrugated fiber sheets can be preferably used.
A corrugated board can also have a flat fiber sheet (liner) in contact with the crests of corrugated fiber sheets (corrugating media)
A corrugated board with a liner can be produced by feeding a fiber sheet destined to be a corrugating medium between two rotating rolls with undulating surfaces to prepare a corrugating medium. The corrugating medium is then sent by a rotating roll which is corrugated on the surface like the corrugation of the corrugating medium, while a fiber sheet destined to be a liner is sent by a roll flat on the surface installed in opposite to the rotating roll. The corrugating medium and the liner are pressed together by both the rotating rolls, to produce the intended corrugated board. The fiber sheet destined to be a corrugating medium can also be a laminate comprising a plurality of fiber sheets.
In the above, if a powdery or fibrous heat fusible polymer is contained in at least either of the fiber sheet destined to be a corrugating medium or the fibrous sheet destined to be a liner, the heat fusible polymer is melted by heat when the corrugating medium and the liner are pressed together by the rotating rolls, to effectively integrate the corrugating medium and the liner.
The heat fusible polymer is only required to be a polymer with a melting point lower than that of the fibers mainly used in the fiber sheet, for example, polystyrene or poly-xcex1-olef in forming the ion exchange fibers, and is not especially limited. It can be selected as required from low melting point polymers such as polyesters, polyolefins, vinyl polymers, etc. The form of the heat fusible polymer contained in the fiber sheet is not especially limited. However, considering the easiness of mixing with the ion exchange fibers or active carbon fibers and paper formability, it is preferable that the heat fusible polymer is fibrous.
A filter medium with an adsorption capacity of 300 eq/m3 or more can be obtained by using chemically modified active carbon fibers (for example, active carbon fibers carrying phosphoric acid), or ion exchange fibers as the fibers forming the fiber sheets.
It is preferable that the ion exchange fibers are based on polystyrene. It is more preferable that they are conjugate fibers consisting of a polymer with ion exchange groups introduced into crosslinked insolubilized polystyrene and a reinforcing polymer (e.g., a polyolefin).
Since polystyrene has low toughness, it is difficult to form fibers with practically endurable mechanical performance by polystyrene alone. So, for obtaining fibers with mechanical properties, a reinforcing polymer is used to be conjugated with polystyrene.
It is preferable that the water content of the ion exchange fibers is in a range of 1.0 to 5.0. The water content is obtained by immersing the sample fibers as a Na type (Cl type) cation (anion) exchanger sufficiently in ion exchange water, removing the water on the surfaces by centrifugal dehydration, measuring the weight (W) immediately, drying the sample, measuring the weight (W0) after drying, and calculating from the following formula using the measured values. Water content=(Wxe2x88x92W0)/W0.
It is preferable that the diameter of the ion exchange fibers is in a range of 15 to 100 xcexcm in a dry state. A more preferable range is 20 to 70 xcexcm, and a further more preferable range is 30 to 50 xcexcm. If the diameter is in this range, the specific surface area as a fiber sheet can be enhanced.
The conjugated form of the polymer to have ion exchange groups introduced and the reinforcing polymer is not especially limited. Core-sheath type conjugate fibers consisting of an ion exchange polymer as the main sheath component and a reinforcing polymer as the core component can be preferably used. Islands-in-a-sea-type conjugate fibers belong to the core-sheath type conjugate fibers, and are especially preferable since they have high toughness and high bondability between the polymer to have ion exchange groups introduced and the reinforcing polymer. Blend type conjugate fibers in which both the polymers are blend-spun can also be preferably used.
Polymers which can be used as the reinforcing polymer include poly-xcex1-olef ins, polyamides, polyesters, polyacrylic compounds, etc. Among them, poly-xcex1-olefins are preferable since they are excellent in chemicals resistance.
The poly-xcex1-olefins include polyethylene, polypropylene, poly-3-methylbutene-1, poly-4-methylpentene-1, etc. Among them, polyethylene is preferable since it is excellent in strength and production convenience.
The fibers are cut into short fibers with an appropriate length, and have ion exchange groups introduced into the ion exchange polymer portions. The ion exchange groups can be either cation exchange groups or anion exchange groups.
The length of the short fibers is optional. If they are too short, fibers are likely to come off from the formed fiber sheet unpreferably, and if too long, the ion exchange reaction in the fiber sheet is likely to be less homogeneous unpreferably. It is preferable that the length of the short fibers is in a range of 0.1 to 10 mm. A more preferable range is 0.3 to 5 mm, and a further more preferable range is 0.3 to 1 mm.
Preferably used anion exchange groups include strong basic anion exchange groups obtained by treating a haloalkylated compound using a tertiary amine such as trimethylamine, and weakly basic anion exchange groups obtained by treating using a secondary or primary amine such as isopropylamine, diethylamine, piperazine or morpholine. To preferably achieve the object of the present invention, a strongly basic anion exchange group is more preferable.
If a filter medium composed of anion exchange fibers is used, the adsorption capacity of the present invention is the anion exchange capacity.
Preferably used cation exchange groups include sulfonic acid groups, phosphonic acid groups, carboxylic acid groups and aminocarboxylic acid groups such as iminodiacetic acid groups.
The ammonia and amines such as organic amines in the raw gas (air) exist as plus charged particulates, mist and gas. To effectively adsorb them for removal using the filter unit of the present invention, it is preferable that the ion exchange groups are cation exchange groups, and sulfonic acid groups are more preferable.
When a filter medium composed of cation exchange fibers is used, the adsorption capacity of the present invention is the cation exchange capacity.
Ion exchange fibers can be produced, for example, by crosslinking and insolubilizing the polystyrene portions of conjugate fibers consisting of a polstyrene based compound and a poly-xcex1-olefin by formaldehyde in the presence of an acidic catalyst, and introducing designed ion exchange groups into the polystyrene portions.
It is preferable that the amount of ion exchange groups introduced is 0.5 meq/g or more based on the dry weight of fibers, and a more preferable range is 1.0 to 10 meq/g.
The housing of the filter medium is, for example, a box with a cover. The top of the cover and the bottom of the box have openings to allow the passage of the gas. The cover is coupled with the box after the filter medium is installed in the box.
It is preferable that the filter medium is installed in the housing in a compressed state, and that its resiliency for restoring its original form is used for immobilizing the filter medium in the housing, without using any adhesive.